As the cost of shrinking semiconductor devices continues to increase, alternative approaches, such as extending the integration of circuits into the third dimension or semiconductor substrate stacking, are being explored. Two or more substrates are bonded together to form a three dimensional structure. Several bonding processes have been implemented for these structures.
Adhesive bonding and dielectric fusion bonding are bonding processes that bond dielectric layers together. Adhesive bonding and dielectric fusion bonding are typically used in processes that require further processing steps after bonding, such as through via etch processes. Conventional direct metal bonding is another method of bonding that bonds metal from one substrate to metal of another substrate. Many conventional direct metal bonding schemes employ solder balls.
Conventional direct metal bonding may cause structural and electrical defects when employed for Cu to Cu bonding on structures that comprise a low k dielectric. As semiconductor chips have scaled, the insulating dielectrics between metal layers have thinned to the point where charge build up and crosstalk adversely affect the performance of the device. Replacing silicon dioxide or like dielectric with a low k dielectric of the same thickness reduces parasitic capacitance, enabling faster switching speeds and lower heat dissipation. However, low k materials are typically porous materials that may not be as mechanically robust as traditional dielectrics.
In conventional direct metal to metal bonding, an additional plasma pre-treatment process may be used to remove surface oxide from the metal surfaces of substrate metal bond pads, in contrast to a less aggressive plasma pre-treatment found in conventional processing. The substrates may then be transferred to a bonding tool. In transferring the substrates, the metal bond pads are exposed to atmosphere. The surface oxides and contamination may begin to accumulate on the metal surfaces of the substrates. Further, the metal bond pads are exposed to atmosphere in the conventional bonding tool. The conventional bonding tool may require a high temperature such as 400° C. for 3D IC bonding. The bonding tool may also apply pressure to the substrates of up to about several psi, as ultrasonic bonding takes place.
A disadvantage of the conventional direct metal bonding is that an additional plasma pre-treatment may damage the device or low k material. Additionally, the relatively high temperature of the conventional bonding may further damage low k material. Damaged low k material may have a higher dielectric constant and thus result in higher RC delay for the 3D device.
A further disadvantage of the conventional direct metal bonding is that the surfaces of the metal pads may have a gap separating them that has a thickness of between about 20-40 μm in a solder ball process. Under the known direct solder bonding process the bonding environment is open to atmosphere, thus the metal pad or solder re-oxidizes in the atmosphere. The metal bond pads from each of the substrates are then bonded with the re-oxidation layers between them. This oxide and/or contamination layer may be porous and moisture may then corrode the metal pads, causing device reliability problems.